1. Field of the Invention
The present invention relates to a device for generation of signals with diversified power levels, especially for pulse formation.
2. Description of the Related Art
A transmitter of a TDMA (Time Division Multiple Access) system is usually conceived in such a manner that it is possible to transmit in individual time slots high frequency signals with varying output or power levels. In this manner the transmission power is set such that in every time slot, each of which is assigned to a respective radio channel, each respective transmission power level is set in such a way that, in spite of diverse distances between the base station and the mobile stations, transmission without interference becomes possible. Since each radio channel is operated with a minimum transmission energy, then because of reduced interferences in the same channel and in adjacent channels in the overall system, a minimized level of interference results. Especially with GSM transmitters there is the requirement that a transmission signal or transmission burst in the burst plateau must be adjustable in defined power steps at a very tight tolerance. Beyond that the edges of the burst must also be contained within a defined amplitude frame, with a simultaneous requirement that the interference spectrum caused because of the steepness of the edge of the burst must be contained within an admissible range. An inadmissible interference spectrum would lead to interferences with neighboring transmission and reception channels.
With a high frequency transmitter with regulated output power, known from DE 39 40 295 A1, a part of the output power of a high frequency power amplifier is coupled out by means of a hybrid coupler located at its output. The coupled out high frequency (HF) power is first rectified and inverted and sent to a servo amplifier as the actual value. By means of a set point adjuster, in accordance with a desired output power, an electrical voltage is applied to the servo amplifier. Using the difference in voltage between the desired value and the actual value, the servo amplifier generates an adjustment voltage which is conducted to an HF amplifier via a control input. The output of the HF amplifier or preamplifier is connected to the input of an HF final stage (power amplifier), so that in this way a closed control circuit originates. However, the expense for realizing such a control circuit is especially high because it is necessary to measure and compensate a great number of parameters to guarantee a sufficiently high dynamic range of the control circuit for all operating conditions. It is especially necessary to include several parameters for the compensation of the nonlinearity and temperature drift of the transmission amplifier.
The customary high frequency power amplifiers have, as a rule, a non-linear transfer function, so that an input signal is not amplified in proportion to its power, at least over a wide range of levels. Likewise, amplifiers with an additional control input for adjusting the amplification have a non-linear dependence of amplifier output power on the control or setting voltage. Thus a precise setting of defined power steps or power levels with a low tolerance, especially for bursts of high dynamic [range] is only possible at considerable expense.
The present invention provides a device for pulse formation of high frequency signals, especially for generating bursts, which makes possible precise setting of diverse levels of power at high burst dynamics at especially low expense. Two separate servo components are provided to set a power step of a pulse- or burst-plateau on one hand, as well as to set a number of level values for the formation of the respective pulse edges on the other hand. For setting the level, preferably a stepwise adjustable attenuator is applied, while for the formation of the pulse edges a controllable amplifier is expediently used.
The invention is based on the consideration that in the generation of high frequency bursts of high dynamics a control loop can be dispensed with if the different power levels of the burst- or pulse-plateau on one hand, and the power during the burst- or pulse-edges on the other hand, are set independently of one another. With TDMA-systems, such as e.g. GSM, there exists a high demand for accuracy with regard to the power level during the burst plateau. Against that, the setting of the power within the burst edges is of subordinate importance as long as the monotonicity of the transfer characteristics and the maintenance of the admissible spectrum is assured. Altogether, by the suitable combination of two servo components it is possible to realize an adjustment range of more than 90 dB. Out of this, depending on the linearity of the controllable amplifier, a plateau dynamic range of over 50 dB can be set with high absolute accuracy.
The first servo component which carries out the precise setting of each power level required within the burst plateau, is in a preferred form of execution a digital step attenuator with a number of binary steps. Through this a large number of attenuation steps are realized in small steps of attenuation. For the second servo component, which takes over the relatively inexact control of power variation change during the burst edges, a controllable power amplifier is used. The input power level of this power amplifier is suitably chosen in a manner such that its output power at minimum attenuation of the stepwise attenuator connected, as the first servo component, at the input side [of the power amplifier] is located only slightly below its 1 dB compression point. Since customary amplifiers, which operate within an operating point range, have below the 1 dB compression point a linear range of amplification of more than 45 dB, corresponding input signals, which are located within this linear dynamic range are transferred to the output with a small relative error. With the MMIC (Monolithic Microwave Integrated Circuit) amplifiers available in today""s market this error in linearity amounts to less than xc2x11 dB over a dynamic range of almost 50 dB below the 1 dB compression point. Consequently, the signal levels supplied at the input of the amplifier within this range by the first servo component are preserved in their relative separations at the output of the amplifier. Only the levels are raised corresponding to the amplification of the amplifier.
The control input of the controllable amplifier serves for lowering the burst level below the lowermost plateau level. Through a correspondingly controlled voltage the output power of the amplifier is raised or lowered according to the required steepness of the edge. Normally, no special accuracy of the level is required there. This inexact setting range of the controllable amplifier can, if the customary components are used, amount up to about 50 dB.
This joint operation of the two servo components on the high frequency signal or on its bursts is controlled by a CPU, for example via D/A converters (DAC, Digital-Analog Converter). Thus, for example, a first D/A converter, coordinated with the first servo component, can control the 21 output steps specified in a GSM transmitter in 2 dB steps, while a second D/A converter, coordinated with the second servo component, forms, through the output of some few values, the pulse edges of the respective bursts. To this end the second D/A converter controls the second servo component, realized as an amplifier, upwards or downwards between saturation and minimum power, with a low pass filter preferably being connected after the single, or after every, D/A converter for pulse smoothing purposes. Upon application of one D/A converter each for each of the control elements, the corresponding low pass filters have different time constants.
To further amplify the output power of the controllable amplifier serving as the second servo component, a power amplifier (final stage) can be connected after it. This final stage, which for example can be a fixed [gain] amplifier, is likewise operated, analogously to the second servo component, in the range below saturation power, so that the power set in the pulse forming device connected before it is further amplified in proportion. Thereby the linearity of the level setting remains preserved, so that each respective power level, and thereby the burst plateau, can be set. If a final stage in form of a likewise adjustable power amplifier is employed, the second servo element works as preamplifier.
The advantages gained by the invention consist especially in that TDMA bursts can be generated at very favorable cost in a simple manner and with high dynamics. Using customary structural components it is thereby possible to realize a burst dynamic of over 90 dB at a low cost.
The device can always be applied in such cases where pulse formation of high frequency signals of large dynamic [range] with application of simple and inexpensive means is desired or required. Thus it is especially suitable for application in TDMA transmitters within the GSM system, because, on one hand, by applying an attenuator as first servo element in small, defined power steps, e.g. in a 2 dB raster (ramping), the power of a burst of a high frequency signal can be set within a time division multiple access system (TDMA) from one time slot to the next time slot with high accuracy. On the other hand it is possible by means of the second servo element, in the form of a controllable amplifier, to master the edge steepness even of a burst of high dynamic, so that an admissible interference spectrum is maintained, and interferences through neighboring radiations are prevented.